The virtues
of the mosquito fish, Gambusia affinis, have been extolled by many mosquito
control organizations. However, the species can be a major pest when introduced
outside their natural range. In addition to eating mosquito larvae they eat
a variety of other insect larvae, zooplankton and aquatic plants. They are
aggressive and have been known to feed on eggs, larvae, and juveniles of
various native fishes and amphibians. Because of these negative impacts,
Gambusia is a regulated species in Washington State, and cannot be introduced
without a fish stocking permit. Under current WDFW policy, transfer/stocking
permits may only be issued to organized Mosquito Control Districts, the U.S.
Army Corps of Engineers, and city, county or state health departments, they
may not be issued to private individuals. It is prohibited to stock Gambusia
in Clallam, Jefferson, Kitsap, Grays Harbor, Mason and Thurston and portions
of Lewis County that drain into the Chehalis river to protect the Olympic
mud minnow.